In recent years, regarding high electric power amplifiers used in base stations for radio communication systems and in heating apparatuses using microwaves, there have been stepped-up technical developments for high-efficiency compound power semiconductor devices which have high withstand voltages and high electric power densities and are capable of high-speed operations and are made of wide-gap semiconductors such as silicon carbide (SiC), gallium nitride (GaN), for example. As a result, energy saving is advanced in the high electric power amplifiers.
In cases of using such compound power semiconductor devices for high-speed operations with larger electric power, in general, depression-type device structures have been employed, in order to realize higher electric power densities for providing larger output amplitudes.
Such a depression-type FET has a normally-ON characteristic, which necessarily necessitates applying a gate bias voltage which is a negative voltage to the gate terminal thereof. If a drain voltage is applied to the drain terminal in such a depression-type FET, in a state where no gate bias voltage is applied to the depression-type FET, this causes a larger electric current to flow through the depression-type FET, which induces damages of the depression-type FET, thereby causing breakdowns thereof at the worst.
In order to prevent damages or breakdowns of the FET when a drain voltage is applied to the drain terminal in a state where no gate bias voltage is applied thereto, as described above, there is provided a comparator adapted to make a comparison between a reference voltage and the gate voltage applied to the gate electrode in the electric power amplifier for determining whether or not the gate voltage is normal, and, further, there is provided a switch for applying the drain voltage to the drain electrode in the electric power amplifier. In activating the electric power amplifier as described above, at first, a negative-voltage generator for generating a gate voltage is activated, further, the comparator is caused to recognize that the gate voltage has reached a predetermined negative electric potential and, thereafter, the switch is turned on, thereby applying the drain voltage to the drain electrode in the electric power amplifier. The technique described above is disclosed in Patent Literature 1.
According to the technique disclosed in Patent Literature 1, in the event of defects of the circuit for creating the negative voltage to be applied to the gate electrode, the application of the voltage to the drain electrode is automatically inhibited, which protects the electric power amplifier against breakdowns in the event of defects of the creation of the negative voltage.
Further, Patent Literature 2 discloses a technique as follows. There are provided an opening/closing means between a power supply and an electric power amplifier, and a charging circuit having a predetermined time constant. In applying the power supply thereto, the opening/closing means is controlled to be brought into an opened state, further, when the negative voltage has reached a predetermined voltage, the charging circuit starts being charged, and when the charging voltage in the charging circuit has reached a predetermined voltage, the opening/closing means is brought into a closed state.
According to the technique disclosed in Patent Literature 2, in applying the power supply thereto, the power supply voltage is not applied to the electric power amplifier, and after the elapse of the time taken for the negative voltage to reach the predetermined voltage, and the time taken for the voltage charged in the charging circuit to bring the opening/closing means into the closed state, the power supply voltage is applied thereto. This prevents excessive electric currents from flowing through the electric power amplifier, thereby preventing breakdowns of the electric power amplifier.    PLT 1: Unexamined Japanese Patent Publication No. 119-238030    PLT 2: Unexamined Japanese Patent Publication No. 2000-68756